As is well known, polyoxymethylene (POM) resins have been used in various fields as an engineering plastic due to the excellent physical properties (such as mechanical and electrical properties) and chemical properties (such as chemical resistance and heat-resistance properties) that such resins possess. As a result, polyoxymethylene resins have been used as a material to form component parts electric and electronic apparatus, automobiles and other machinery.
Notwithstanding the excellent inherent properties exhibited by polyoxymethylene resins generally, there is still a need for continual improvements to be made in this regard, especially as new and/or more specialized end-use applications for polyoxymethylene resins are identified. For example, some beneficial improvements to the properties of polyoxymethylene resins include: minimizing reductions in the mechanical strength of the resin during extrusion or molding operations; maintaining the color of the resin; reducing mold deposits that are formed during molding, and/or minimizing deterioration of the mechanical properties of the resin caused by heat aging. A principal factor which contributes to all of these aspects of polyoxymethylene resins generally is the propensity of the polymer chain to decompose ("unzip" to use art parlance) when heated.
Polyoxymethylene resin is, in and of itself, relatively easily decomposed when heated in an oxidizing atmosphere or under acidic or alkaline conditions due to its molecular structure. In this connection, the chemically active terminal groups of the polyoxymethylene chain can be stabilized by a variety of techniques so as to prevent (or at least significantly minimize) chain decomposition. For example, for polyoxymethylene homopolymers (i.e., having a linear chain composed solely of repeating oxymethylene units), greater stability can be achieved by esterifying or acetylating the chemically active terminal groups of the polymer chain.
The polyoxymethylene chain itself can be rendered more stable by copolymerizing trioxane, for example, with a monomer having adjacent carbon-carbon bonds such as a cyclic ether or a cyclic formal to yield a polyoxymethylene copolymer comprised mainly of repeating oxymethylene groups which are randomly interspersed with relatively more stable higher oxyalkylene groups (e.g., oxyethylene, oxypropylene, or the like). The resulting polyoxymethylene copolymer can then be hydrolyzed, for example, so as to remove terminal oxymethylene groups such that the resulting polymer has relatively more stable higher oxyalkylene groups at its terminal positions. However, even when a relatively stable, inert polyoxymethylene copolymer is produced, cleavage of the main polymer chain can sometimes occur when the polymer is heated. As a result, incorporating antioxidants and other stabilizing additives into the polyoxymethylene resin is typically an indispensable step to achieve a commercially viable polyoxymethylene resin.
Polyoxymethylene resins which are stabilized by the incorporation of antioxidants and/or other stabilizers are not, however, completly inhibited against decomposition. In fact, even such additive-stabilized polyoxymethylene resins are affected by heat and oxygen in the cylinder of a molding machine when the resin is molded to an extent that the resin chain may decompose to form formaldehyde. As a result of formaldehyde formation, the ambient environment of the molding machine can be polluted. In addition, a tarry mold deposit can form by virtue of polymer degradation which significantly reduces the efficiency of the molding operation as well as detrimentally affecting the surface condition of the resulting molded articles.
Heat aging of polyoxymethylene resin occurs when the resin is subjected to heat for prolonged periods of time. The consequence of heat aging is that the main polymer chain decomposes to an extent that the mechanical strength properties of the resin decrease. Thus, efforts have been attempted in this art to find a more effective stabilization additive and/or additive formulation. For example, hindered phenolic and amine compounds (i.e., sterically hindered phenols and hindered amines) have been added to polyoxymethylene resins in an attempt to improve the stability characteristics of the polyoxymethylene base resin. In addition, other stabilizers, including a combination of polyamides, urea derivatives, amidine compounds, alkali or alkaline earth metal hydroxides and organic or inorganic acid salts have also been employed for the same purpose.
Although hindered phenols are particularly effective antioxidants for polyoxymethylene resins, they are nonetheless weakly acidic which become more acidic due to conversion upon absorption of light. As a consequence, hindered phenols may serve as a polymer decomposition catalyst when used in large amounts thereby exacerbating the polymer's instability. Furthermore, oxidized hindered phenols can contribute to color losses in the polymer, and is a principal factor in the formation of mold deposits.
Thus, the formulation of a stabilizer or stabilization "system" having adequate molding stability, heat aging stability and weather resistance properties is a delicate balance of the attributes and detriments that are imparted to polyoxymethylene resin by any particular stabilizer. It is therefore towards providing polyoxymethylene resin of enhanced stability which minimizes (if not eliminates) the drawbacks associated with conventional stabilizer technologies that the present invention is directed.
Broadly, the present invention is characterized by the incorporation into polyoxymethylene resins of a stabilizing effective amount of specific alkoxy-substituted phenoxy-containing stabilizer compounds which improve the ability of the polyoxymethylene resin to withstand deterioration by oxidation and/or heat as well as minimizing the formation of mold deposits during extrusion. More specifically, the present invention relates to stable, moldable, normally solid polyoxymethylene resin compositions having a stabilizing effective amount of between 0.01 to 5% by weight, based on the polyoxymethylene base resin, of a phenoxy-containing compound having the following general formula (I) or (II): EQU Ar.sup.1 --O--X (I) EQU Ar.sup.2 (--O--X) (II)
where Ar.sup.1 represents an aryl group (excluding one having a hydroxyl group directly bonded thereto as a substituent), such as phenyl, biphenyl or naphthyl groups, and Ar.sup.2 represents a benzene or naphthalene ring (excluding one having a hydroxyl group directly bonded thereto as a substituent), and s is an integer which represents the number of alkoxy-derivative substituents. Preferably, Ar.sup.2 is a benzene ring.
The phenoxy-containing stabilizer may be incorporated into the polyoxymethylene base resin by any conventional technique, but is preferably melt-blended with the polyoxymethylene resin. The polyoxymethylene resin compositions may then be pelletized and used as a feedstock for extrusion and/or injection molding of components or parts associated with a variety of end-use applications.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments thereof.